1. Field of the Invention
This invention relates to electrochemical power sources such as cells, batteries and capacitors. More particularly, the present invention is directed to ambient temperature molten salts that are useful as electrolytes in primary and secondary electrochemical cells and in high energy density electrolytic capacitors. Additionally, such salts are useful as hydraulic fluids and fire retardants.
2. Prior Art
Examples of electrolytes currently being used in rechargeable electrochemical power sources include liquid, gel, and dry polymer types. Dry polymer electrolyte cells without plasticizers exist, but their inadequate conductivity and low lithium ion transference prevent them from being used at ambient or reduced temperatures.
Liquid and gel electrolytes have higher ionic conductivity and adequate lithium ion transference when compared with dry polymer electrolytes. An example is a solvent system of propylene carbonate and 1,2-dimethoxyethane having a lithium salt such as LiPF6 or LiAsF6 dissolved therein. Such an electrolyte is typically used to activate a lithium/silver vanadium oxide (Li/SVO) cell. Additionally, liquid and gel electrolyte cells, such as of a carbonaceous negative electrode and a lithium cobalt oxide positive electrode, are capable of cycling at relatively high rates and low temperatures. One major disadvantage with them, however, is that organic solvents must be included in the electrolyte to improve conductivity and, in the case of the liquid phase, lower viscosity. Liquid and gel electrolytes are also relatively volatile and flammable, which poses a risk of fire when they are heated. In addition, liquid and gel electrolyte cells, whether of a primary or a secondary chemistry, are subject to gassing and subsequent leakage. The packaging and processing required to prevent leakage is complex and, therefore, costly.
In contrast, electrolytes based on ambient temperature molten salts promise the safety of dry polymers along with substantially higher ionic conductivies. One example is described in U.S. Pat. No. 5,827,602 to Koch et al., which relates to derivatives of imidazole and the usefulness of these ambient temperature molten salts as electrolytes for high energy density batteries and capacitors. Pyridine and other five and six membered heterocyclic cations containing one or more nitrogen atoms are also discussed. For example, 1-methyl-3-ethyl-1H-imidazolium, which is shown below, forms molten salts at ambient temperature with several different anions.
